Crosslinked epoxy resins have a mostly very high crosslink density and so have some valuable properties, making them the most widely used polymers alongside thermoplastics. Among these properties are their hardness, strength, chemical resistance and thermal stability. This makes these epoxy resins suitable for applications in a very wide variety of fields, for example for the production of fiber-reinforced plastics, for insulation materials in electrical engineering, in the manufacture of engineering adhesives, high-pressure laminates, stoving enamels and so forth.
Thermosets also have a serious disadvantage which often prevents their use. Because of their highly crosslinked condition, they have very little impact toughness. This applies particularly to the low-temperature range, i.e., temperatures below 0° C., and so thermoplastic polymers are normally preferred for applications where the thermoset might be exposed to high mechanical loads, e.g., impacts, at low temperatures, although the use of thermoplastic polymers does mean that disadvantages associated therewith, such as lower heat resistance and chemical resistance, have to be accepted.
Several methods have been developed to improve the impact toughness or flexibility of thermosets.
Most of these methods have the purpose to incorporate elastic components as impact modifiers in the reactive resins.
It is known to add pulverulent, soft fillers, such as rubber powder or elastomeric powders of polymer, to reactive resins. The particle size of pulverulent additives of this type is in the range from about 0.04 to 1 mm, and this is evidently not sufficient to achieve the desired type of improvement in these reactive resins, and there are also attendant disadvantages for other important performance characteristics of thermosets modified in this way.
Plasticizers are added in an attempt to improve the impact toughness of crosslinked epoxy resins. This does achieve improved impact toughness, but unfortunately at the cost of sacrificing other essential properties of these thermosets. In addition, the use of plasticizers entails a latent risk of exudation after the crosslinking of the reactive resin, with the associated negative consequences for surface properties of the material, such as adherence, coatability or gloss.
It is also known to use liquid or solid but non-crosslinked butadiene-acrylonitrile rubbers (nitrile rubber, NBR) as toughness-improving additives in reactive resins. These elastomers contain functional groups which can be reacted with the reactive resin during the crosslinking step or else in an earlier reaction. Reaction between the reactive resin and the functional groups positioned at the surface of the nitrile rubber particles produces a firm attachment of the rubber phase to the thermoset matrix.
However, these nitrile rubber-modified thermosets unfortunately also have significant shortcomings. For instance, the thermal stability of thermosets becomes inferior on modification with nitrile rubber, and so calls the high-temperature utility of thermosets modified with nitrile rubber into question. The same applies to many electrical properties, such as dielectric strength for example. Because the compatibility of the nitrile rubber with most epoxy resins is relatively good, a certain proportion of the rubber does not participate in the phase separation of crosslinking and becomes incorporated in the resin matrix, impairing the performance profile of the final thermoset. A further disadvantage is the very high viscosity of the nitrile rubber modifiers, which leads to processing problems and impairs the flow properties of the modified reactive resin. U.S. Pat. No. 5,284,938 discloses using siloxane-polyester copolymers as impact modifiers in epoxy resins. Unfortunately, they have the disadvantage of greatly reducing the stiffness of the epoxy resins, which is not always desirable.
EP 0266513 B1 describes modified reactive resins, processes for their production and their use. In effect, functionalized crosslinked organopolysiloxane rubbers are produced in situ in the resin to be used. It is restricted to compositions which, in addition to a reactive resin, include a maximum of 2-50% by weight of three-dimensionally crosslinked polyorganosiloxane rubbers having particle sizes of 0.01 to 50 micrometers in amounts of 2-50% by weight, although the properties of the composition described therein are inadequate in terms of impact strength and impact toughness. Again, the flow properties of the epoxy resin thus modified are adversely affected.
WO2006037559 describes modified reactive resins and also processes for their production. Solutions of preformed particles in organic solutions are mixed with reactive resins and the reactive resins of the invention are recoverable by subsequently removing the solvent. The disadvantage with this process is the fact that the solvent quantities are very large at times and are very costly and inconvenient to remove again, and if they are not completely removed, can lead to defects in the material during the curing of the reactive resins.